Hey @Tiffany37! Unfortunately, most email clients don’t support custom fonts, and there’s no way to enforce it, either. In general, we advise using custom fonts in emails with caution, since many email clients interpret/support styling code differently.
As a result, we are limited in the support we can provide for more advanced template modifications like this. I'm really sorry about that! (That said, to answer your question, we don't actively block the use of custom fonts; it's more likely that the font, or your method of importing it, is not supported by the email client you're testing with.)